Proton exchange membrane fuel cells (PEMFC) have attracted considerable attention as promising power generators for automotive, stationary, as well as portable power, due to their high-energy efficiency and low emissions. The membrane electrode assembly is one of the key components in the design of improved PEM fuel cells.
Prior art related to methods of making membrane electrode assemblies covers issues in the following areas: (i) direct membrane catalyzation, (ii) catalyzation of coated electrode substrates, (iii) need for effecting membrane electrode bonding for seamless proton transport (iv) effective solubility of reactant gases (in particular oxygen), (v) use of pore forming agents for effective gas transport within the electrode structure. Prior art literature relates to the specific objective of enhancing mass transport and the ability to operate a fuel cell on a sustained higher power density level.
In the context of prior art, direct catalyzation of the membrane has been described in various patents and scientific literature primarily on aqueous based polymer electrolytes, most notably of the perfluorinated sulfonic acid type. At the current state of the technology, prior efforts together with current approaches have to be tempered with the ability to translate developments in this regard to mass manufacturability while keeping reproducibility (batch vs. continuous) and cost in perspective. Depending on the deposition methods used, the approach towards lowering noble metal loading can be classified into five broad categories, (i) thin film formation with carbon supported electrocatalysts, (ii) pulse electrodeposition of noble metals (Pt and Pt alloys), (iii) sputter deposition (iv) pulse laser deposition, and (v) ion-beam deposition. While the principal aim in all these efforts is to improve the charge transfer efficiency at the interface, it is important to note that while some of these approaches provide for a better interfacial contact allowing for efficient movement of ions, electrons and dissolved reactants in the reaction zone, others additionally effect modification of the electrocatalyst surface (such as those rendered via sputtering, electrodeposition or other deposition methods).
In the first of the five broad categories using the ‘thin film’ approach in conjunction with conventional carbon supported electrocatalysts, several variations have been reported, including (a) the so called ‘decal’ approach where the electrocatalyst layer is cast on a PTFE blank and then decaled on to the membrane (Wilson and Gottesfeld 1992; Chun, Kim et al. 1998). Alternatively an ‘ink’ comprising of Nafion® solution, water, glycerol and electrocatalyst is coated directly on to the membrane (in the Na+ form) (Wilson and Gottesfeld 1992). These catalyst coated membranes are subsequently dried (under vacuum, 160° C.) and ion exchanged to the H+ form (Wilson and Gottesfeld 1992). Modifications to this approach have been reported with variations to choice of solvents and heat treatment (Qi and Kaufman 2003; Xiong and Manthiram 2005) as well as choice of carbon supports with different microstructure (Uchida, Fukuoka et al. 1998). Other variations to the ‘thin film’ approach have also been reported such as those using variations in ionomer blends (Figueroa 2005), ink formulations (Yamafuku, Totsuka et al. 2004), spraying techniques (Mosdale, Wakizoe et al. 1994; Kumar and Parthasarathy 1998), pore forming agents (Shao, Yi et al. 2000), and various ion exchange processes (Tsumura, Hitomi et al. 2003). At its core this approach relies on extending the reaction zone further into the electrode structure away from the membrane, thereby providing for a more three dimensional zone for charge transfer. Most of the variations reported above thereby enable improved transport of ions, electrons and dissolved reactant and products in this ‘reaction layer’ motivated by need to improve electrocatalyst utilization. These attempts in conjunction with use of Pt alloy electrocatalysts have formed the bulk of the current state of the art in the PEM fuel cell technology. Among the limitations of this approach are problems with controlling the Pt particle size (with loading on carbon in excess of 40%), uniformity of deposition in large scale production and cost (due to several complex processes and/or steps involved).
The use of carbon nanotubes (CNTs) in electrodes for membrane electrode assemblies (MEA) has recently gained much attention (Baughman 2002). Prior art relates to the use of CNT as carbon support for the catalyst particle. CNT can replace carbon Vulcan as catalyst support. At present, all pre-commercial fuel cells use supported Pt and Pt alloys as their electrocatalysts. The critical properties to consider when choosing an electrocatalyst support include its electrical conductivity, surface area, macro-morphology, microstructure, corrosion resistance, and cost. Carbon black (CB), such as Vulcan XC-72, has been the most widely used electrocatalyst support because of its reasonable balance among electronic conductivity, surface area and cost. Recently, many nanostructured carbon materials with graphitic structure, such as nanotubes (CNTs), nanofibers (CNF) nanocoils, nanoarrays and nanoporous hollow spheres, have been studied. Among them, CNTs are of particular interest due to their unique electronic and micro and macro structural characteristics. CNTs have also been shown to be more corrosion-resistant than CB under simulated fuel cell operation conditions.
There are two categories of carbon nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). A SWNT is a graphene sheet rolled into a cylinder. A MWNT is comprised of several coaxially arranged graphene sheets rolled into a cylinder. According to theoretical predictions, SWNTs can be either metallic or semiconducting depending on the tube diameter and helicity. The band gap is proportional to the reciprocal diameter, 1/d. For MWNTs, scanning tunneling spectroscopy (STS) measurements indicate that the conduction is mainly due to the outer shell, which are usually much larger than SWNTs. Therefore, MWNTs have a relatively high electrical conductivity, that's why it is preferred MWNTs to be the support for the platinum catalyst in PEMFCs because of their relatively high electrical conductivity because current growth methods for MWNTs are simpler than those for SWNTs. Apart from the replacement of carbon Vulcan as catalyst support from CNT, CNT can be added into the catalyst ink of the electrode. Prior art relates to the inclusion of CNT in catalyst ink proving that CNTs generally act as paths for electron conduction and efficient gas communication (Schulte 2006). However, one of the main problems encountered with the addition of CNT in a media (i.e. catalyst ink), is their dispersion.
It thus would be desirable to provide a method for improving the low dispersibility on liquid media of the CNTs in the catalyst ink. This could be done by tethering the CNT into a polymer backbone. At the same time this polymer backbone could act as proton conductor bearing polar sites in the main chain. So, the insertion of a multifunctional component into the catalyst ink that could provide pathways for protons and electrons in the vicinity of the catalyst's sites would enhance the kinetics of electrode reactions resulting in higher catalytic activity. This could also result in lower Pt loading, in the range of 0.01 to 0.5 mg/cm2, as compared to the current state of the art, thus providing for a better gravimetric energy density. It would be particularly desirable from the perspective of long term sustained power density as well as better tolerance to both load and thermal cycling (especially transitions to below the condensation zone).